When I tried to send an email on my new iiNet last night, I
discovered that outgoing port 25 was blocked. This is quite common
with ISPs, since it tends to prevent a good deal of spam and other
malware activity. But it’s a nuisance for me, since I like to send
mail via my own mail server.

But, unlike many other ISPs (not looking at anyone cough bigpond in
particular), iiNet let you turn off the blocking if you know what
you’re doing.